Chemistry 101
Experiment 5 - DIFFUSION OF GASES AND GRAHAM'S LAW
In this experiment, the relative rates of diffusion of two gases will be determined. Rates of
diffusion yield information that can lead to calculation of the molecular weights of gases.
Gases consist of particles that are in constant rapid motion. This motion causes gases to
travel across space (diffuse) and completely mix with each other. It is this diffusion that
eventually causes one to notice smells such as perfume, fish, ammonia, etc.
In this experiment, the rates of diffusion of two gases, ammonia (NH3) and hydrogen
chloride (HCl), will be investigated. These gases are convenient to use for such an experiment
because, when they meet and react, they form a white smoke consisting of ammonium chloride
(NH4Cl):
NH3(g) + HCl(g)  NH4Cl(s)
Therefore, if ammonia gas and hydrogen chloride gas are released simultaneously at opposite
ends of a glass tube, a white ring of smoke will form at the location where they meet.
This experiment will demonstrate rates of diffusion, a property of gases investigated by
Thomas Graham. In 1829, he proposed his law of diffusion which states that the rate of diffusion
of a gas is inversely proportional to the square root of its density:
1
R
d
However, since the ideal gas law indicates that the density of a gas and its molecular weight are
proportional, we can write:
1
R
MW
If the rates of diffusion of two gases are compared, this yields the following equations:
R1
R2
=
1
MW1
or
1
MW2
R1
=
R2
MW 2
MW1
Thus, if the rates of diffusion of two gases are known and the molecular weight of one of them is
known, the molecular weight of the other gas can be calculated:
2
R1 ( MW1)
MW 2 =
2
R2
In this experiment, the distance each gas travels will be measured as well as the time it
takes for them to meet and react (D = distance, t = time):
D NH3
R NH3
R HCl
t
=
DHCl
=
MW HCl
MW NH3
t
This can be rearranged to:
MW HCl =
2
R NH3 MW NH3
2
R HCl
PROCEDURE
Wear your safety glasses while doing this experiment.
You will carry out the experiment with a partner.
Obtain a gas diffusion apparatus from the stockroom, consisting of a glass tube and two
corks which have holes drilled in them and a stopwatch. Clamp the glass tube horizontally on a
ring stand (See Figure 5). Loosely place a wad of cotton into each cork and in the hood saturate
one wad of cotton with drops of concentrated NH3 solution and the other wad of cotton with
drops of concentrated HCl solution. Be sure to keep the corks far apart to avoid a premature
reaction.
Figure 5: Gas Diffusion Apparatus
Once the corks are ready, return to your lab bench and insert the corks simultaneously into
the opposite ends of the glass tube. One partner will begin to measure the time. Now, very
carefully observe the glass tube and at the time of the first appearance of a white ring of
ammonium chloride, record the time and mark the location of the ring on the tube.
Measure and record the distance each gas traveled.
To repeat the experiment, clean the glass tube with tap water. Rinse with deionized water
and then with alcohol. Dry the tube completely, then clamp the tube horizontally, once again.
Discard the cotton wads in the waste beaker in the hood and then place clean cotton in the
corks and saturate as before, then repeat the experiment.
Chemistry 101
Experiment 5 - DIFFUSION OF GASES AND GRAHAM'S LAW
Section_____________
Name_____________________
Pre-Laboratory Assignment
1. A gas of unknown molecular weight is found to diffuse at a rate of 0.19 cm/s compared to
0.59 cm/s for helium gas. Calculate the molecular weight of the unknown gas.
2. Which of all of the possible gases should have the highest rate of diffusion?
3. A balloon inflated with helium gas will rise to the ceiling of a room. However, after a certain
period of time, the balloon will descend to the floor. Why does the balloon eventually descend?
4. If the cork containing the concentrated NH3 solution were inserted into the glass tube several
seconds after the insertion of the cork containing HCl, what effect would this have on the
calculated value of the molecular weight of HCl?
Chemistry 101
Experiment 5 - DIFFUSION OF GASES AND GRAHAM'S LAW
Section_____________
Name_____________________
Report Sheet
TRIAL
First
Second
Third
Time of insertion of stoppers
______
______
______
Time of appearance of smoke
______
______
______
Total time elapsed (seconds)
______
______
______
Distance travelled by NH3(cm)
______
______
______
Distance travelled by HCl(cm)
______
______
______
Rate of diffusion of NH3(cm/s)
______
______
______
Rate of diffusion of HCl(cm/s)
______
______
______
17.0
17.0
17.0
______
______
______
MW of NH3
Calculated MW of HCl
Mean MW of HCl
True value of the MW of HCl
Percent error of mean value
Show your calculations below:
______
36.5
______
Chemistry 101
Experiment 5 - DIFFUSION OF GASES AND GRAHAM'S LAW
Post-Laboratory Questions
1. How many times faster will CH4 gas diffuse compared to C4H8 gas?
2. If CH4 gas and C4H8 gas are released simultaneously at the left and right ends respectively of
a 50.0 cm long glass tube, at what distance from the left end of the tube will they meet?
3. Methane gas, CH4, diffuses 2.3 times faster than an unknown gas at the same temperature and
pressure. What is the molecular weight of the unknown gas?
4. Does the first appearance of the white smoke indicate the first contact of the NH3 and HCl
molecules? Explain your answer. If the answer is “no”, how will this affect the calculated value
of the molecular weight of HCl?